This invention relates to a circuit arrangement for supplying a lamp with a DC lamp current comprising only passive components and equipped with
mains input terminals for connection to poles of a supply voltage source,
rectifying means, equipped with two input terminals, coupled to the mains input terminals, and two output terminals, for generating a DC voltage out of a low frequency AC voltage supplied by said supply voltage source,
capacitive means, coupled between the mains input terminals and the output terminals of the rectifying means,
a load circuit coupled to the output terminals of the rectifying means and comprising terminals for connection to the lamp, and
inductive means.
The invention also relates to a compact lamp.
A circuit arrangement as mentioned in the opening paragraph is known from U.S. Pat. No. 4,929,871. In the known circuit arrangement the inductive means and the capacitive means are arranged in series between one of the mains input terminals and one of the input terminals of the rectifying means that are formed by a diode bridge. The load circuit contains only the terminals for lamp connection. During operation the known circuit arrangement supplies a DC current to the load. The known circuit arrangement is relatively simple and therefore relatively cheap. An important disadvantage of the known circuit arrangement, however, is that during stationary operation the lamp current becomes zero in every half period of the low frequency AC voltage. As a result the lamp will extinguish in every half period of the low frequency supply voltage. In case the lamp is a discharge lamp, it will therefore have to be reignited in each half period of the low frequency supply voltage. This reduces the life of the lamp electrodes and also causes reignition losses.
The invention aims to provide a circuit arrangement for operating a discharge lamp (low pressure or high pressure) that is relatively cheap and simple and does not have the disadvantages mentioned hereabove.
A circuit arrangement as mentioned in the opening paragraph is therefore, in accordance with the invention, characterized in that the load circuit is equipped with a series arrangement connecting the output terminals of the rectifying means and comprising the inductive means and the terminals for connection to the lamp.
Since the inductive means are in series with the lamp during lamp operation, the inductive means supplies the lamp with a xe2x80x9ckeep-alive currentxe2x80x9d when the voltage between the input terminals of the rectifying means is close to zero Volt. This xe2x80x9ckeep-alive currentxe2x80x9d flows from a first terminal of the inductive means through the rectifying means and the lamp to a second terminal of the inductive means. The operation is called continuous operation, if the circuit arrangement is so dimensioned that the lamp current never drops to zero. Therefore, in case of continuous operation and in case the lamp is a discharge lamp, the lamp does not extinguish and need not be reignited every half period of the supply voltage. It should be mentioned that the circuit can also be so dimensionedntat so the keep alive current becomes zero in the time lapse during which the voltage between the input terminals of the rectifying means is close to zero. This is called discontinuous operation. The lamp, if it is a discharge lamp, will extinguish and will have to be reignited every half period of the supply voltage. It has been found both for continuous as well as for discontinuous operation that the inductance of the inductive means and the capacitance of the capacitive means in a circuit arrangement according to the invention can be chosen considerably smaller than in the circuit arrangement disclosed in U.S. Pat. No. 4,929,871. The circuit arrangement according to the invention can therefore be considerably less voluminous and weigh substantially less than the known circuit arrangement. Another important advantage over the prior art that was found to exist for continuous operation only is a fast ignition of the discharge lamp without xe2x80x9cpinkingxe2x80x9d and with only relatively little sputtering of emitter material from the electrode taking place. Since the inductive means are only subjected to a DC current, the flux swing and the core losses in the inductive means are relatively small resulting in an efficient operation of the circuit arrangement.
Preferably, the impedance of the capacitive means is higher than the impedance of the inductive means. The capacitive means functions as a current limiter, while the inductive means supplies current to the lamp during at least part of the time lapse during which current is not supplied to the lamp by the supply voltage source.
Since, as explained hereabove, the inductance of the inductive means can be relatively small, the inductive means may comprise a choke with a core of compressed iron powder. The inductive means can thus be realized in a relatively cheap and simple way and the core can easily be manufactured in many different forms. This latter aspect can be very important in the case where the circuit arrangement is integrated in a compact lamp, since it is possible to choose a form of the core, such as for instance a toroid, that will easily fit into the compact lamp and provides good heat conduction from the circuit arrangement into the environment.
The rectifying means may comprise a diode bridge but alternatively the rectifying means may comprise a voltage doubler. This is particularly usefull in case the amplitude of the low frequency supply voltage is relatively low. In case the capacitive means is formed by two capacitors comprised in the voltage doubler, these two capacitors function both as current limiter and as part of the voltage doubler, so that different functions in the circuit arrangement are realized using relatively few components. The rectifying means can also comprise both a diode bridge and a voltage doubler, the voltage doubler comprising two of the diodes of the diode bridge and two capacitors. The rectifying means substantially function as a voltage doubler at low loads, i.e. before the lamp operated by means of the circuit arrangement has ignited. This way a high enough ignition voltage is realized, even in the case where the amplitude of the low frequency AC voltage supplied by the supply voltage source is relatively low. After ignition of the lamp, when the load has increased, the rectifying means substantially function as a diode bridge.
Good results have been obtained with relatively simple embodiments of the circuit arrangement according to the invention wherein the circuit arrangement exclusively comprises components that make up the capacitive means, the inductive means and the rectifying means.
Since it comprises relatively few components and is therefore relatively small, a circuit arrangement according to the invention is very suitable to be used in the ballast means of a compact lamp comprising
a light source provided with a vessel that is closed in a gastight manner and transmissive for visible radiation,
a housing connected to the light source and provided with a lamp cap,
ballast means electrically connected to the light source for operating the light source and positioned at least partly in a space surrounded by the housing.